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<title>Taxonomy of Categories</title>
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	<h1>Taxonomy of Categories</h1>
	
	Facets are defined using a hierarchy of categories, known as a <i>Taxonomy</i>.
	For example, the taxonomy of a book store application might have the following structure:
	<ul>
		<li>Author
			<ul>
			<li>Mark Twain</li>
			<li>J. K. Rowling</li>
			</ul>
		</li>
	</ul>
	<ul>
		<li>Date
			<ul>
			<li>2010
				<ul>
				<li>March</li>
				<li>April</li>
				</ul>
			</li>
			<li>2009</li>
			</ul>
		</li>
	</ul>
	
	The <i>Taxonomy</i> translates category-paths into interger identifiers (often termed <i>ordinals</i>) and vice versa.
	The category <code>Author/Mark Twain</code> adds two nodes to the taxonomy: <code>Author</code> and 
	<code>Author/Mark Twain</code>, each is assigned a different ordinal. The taxonomy maintains the invariant that a 
	node always has an ordinal that is &lt; all its children.
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